If they cannot hear it and it had stopped then how do they know what it was?
Because they're probably asked about the noise on every flight.
I think those of us who fly frequently are pretty familiar with the usual bumps and grinds of an aircraft and are able to notice something genuinely unusual. This is especially true of PPruNers since we all have a higher than average interest in aviation! If pax have genuine concerns they should not be made to feel stupid for raising them with the cabin crew. After all, its their job to ensure that we are comfortable and safe.
Once while flying at cruising altitude (cant remember who with or where to) lots of dry ice 'smoke' started pouring out from the air conditioning ducts. Cue lots of concerned looking faces and rambling chit chat. Quick as a flash came a p.a. announcement explaining what the 'smoke' was and assuring everyone that there was nothing to worry about. A small gesture but it made a big difference to a lot of people. Even if the 'smoke' was a regular occourance on many flights, a quick explaination would be no more intrusive and repetitive to crew and frequent flyers than the saftey demonstration.
The strangest thing of all about this is that as passengers we are told to be extra vigilant at airports these days. Watch out for unattended bags, look out for suspicious people etc etc... Why shouldn't that vigilance stretch to aircraft saftey?